As is well known in the pertinent art, superconductors can be relatively easily manufactured as a ceramic for use in several applications. However, since a small diameter superconductor lacks sufficient inherent strength to be handled easily, it must be supported by a substrate during its use. Furthermore, a substrate is needed during the manufacture of the ceramic superconductor to provide a base on which the unsintered grains of superconductor material can be coated. Unfortunately, the extreme changes between manufacturing temperatures and operational temperatures affect both the chemical and the physical interaction between the substrate and the ceramic. As used here, chemical compatibility means an essentially inert relationship wherein there is little, if any, interdiffusion between the ceramic superconductor and the substrate. With this definition, chemical compatibility between the substrate and the superconductor is important for at least two reasons. First, the diffusion of substrate constituents into the superconductor material will dope the superconductor with foreign elements which invariably poison the superconductor. The result is a superconductor which has a lower critical temperature (T.sub.c) and also a reduced critical current density (J.sub.c) Second, the diffusion of superconductor material from the superconductor into the substrate may change the desirable characteristics of the substrate, e.g. embrittle the substrate. Thus, interdiffusion should be prevented, or at least minimized. One solution is to select substrate materials having minority constituents which form oxide shells such as those disclosed in co-pending patent application Ser. No. 265,827 for an invention entitled "Substrate for Ceramic Superconductor" and our co-pending application for an invention entitled "Substrate for Ceramic Superconductor with Improved Barrier," both of which are assigned to the same assignee as the present invention. Another solution is to select a substrate material which is inherently chemically compatible with the superconductor material. With this characteristic in mind, it has been determined that one such material is nickel
(Ni) Pure nickel (Ni), however, is more magnetic at the cryogenic operating temperatures of superconductor materials than is desirable for some contemplated applications. Further, nickel (Ni) is more magnetic than desirable at the room temperature conditions which may be used for magnetic grain alignment fabrication of the ceramic superconductor, as disclosed in our copending U.S. patent application Ser. No. 289,968, assigned to the same assignee as the present invention.
Physical compatibility between the substrate and the superconductor is also important. Specifically, the substrate must be able to support the superconductor ceramic without putting undue stresses on the ceramic. This requires consideration of the respective coefficients of thermal expansion of the substrate and the superconductor, as well as the stability of the substrate. Moreover, these considerations apply over a very wide temperature range and a variety of operating configurations. Insofar as the respective coefficients of thermal expansion are concerned, it is preferable that the ceramic be placed in compression when the combination of substrate and ceramic superconductor is cooled.
The magnetic properties of the substrate are also of great importance. Particularly, there is a need for a nonmagnetic or weakly magnetic substrate when the superconductor is to be used as a magnet winding. Otherwise, the magnetic field generated by the magnet windings can be distorted. Also, there is a need for a nonmagnetic substrate if the substrate is to be coated with grains of superconductor material by an electrophoresis process such as is disclosed in our co-pending patent application Ser. No. 289,968 for an invention entitled "Apparatus and Method for Manufacturing a Ceramic Superconductor Coated Metal Fiber," and which is assigned to the same assignee as the present invention. This is so in order not to perturb the grain aligning magnetic field in the vicinity of the substrate wire and thus prevent a noncircular coating.
In light of the above, it is an object of the present invention to provide a substrate for a ceramic superconductor which is inherently substantially chemically compatible with the ceramic. Another object of the present invention is to provide a ceramic superconductor substrate which can support the ceramic without placing undue stresses on it. Yet another object of the present invention is to provide a ceramic superconductor substrate which is substantially more chemically compatible with the superconductor than is pure nickel. Still another object of the present invention is to provide a substantially nonmagnetic ceramic superconductor substrate. Yet another object of the present invention is to provide a superconductor that is relatively easy to manufacture and comparatively cost effective.